This invention relates to the field of pre-press proofs and in particular relates to a method of protecting the surfaces of such proofs by applying a thin clear hard plastic overlay thereto by means of a pressure sensitive adhesive adhered to the back surface of the overlay.
Presently there are two commercially accepted methods of making pre-press or pre-plate proofs. These are the CROMALIN system of DuPont and the TRANSFER-KEY system of 3M Company. These are used by color separation houses which make separate positives or negatives for each color in a four-color printing process for delivery to the printer. These also are used by publishers, platemakers, pressman, and, of course, by customers who can see a full color proof without the expense by making plates and press runs.
In the DuPont process, a film consisting of a photopolymer sandwiched between cover and base sheets is used. The base sheet is removed and the photopolymer and cover sheet are laminated to a sheet of proof stock.
A separation halftone film of the appropriate color is registered to the proof sheet and the sensitized proof sheet is exposed through the film in an ultraviolet-rich source.
The light sensitive photopolymer layer of the film is colorless and tacky as manufactured. The halftone image of the separation positive permits the exposure to selectively harden the non-image areas of the photopolymer. These exposure-hardened areas lose their tacky characteristic so the film accepts the color toner only in the unexposed image areas.
The exposed laminate is placed on a toning console, the cover sheet is peeled off, and the first dry color toner is applied.
A separate acrylic applicator is used for each color toner. The tacky photopolymer accepts toner reproducibly with a standardized toning pattern and procedure. The toning console is equipped with a suction downdraft around the toning platform to keep color toner dust out of the air.
The process is repeated: laminate, register, expose, and tone, for each separation film. After all of the images have been toned--yellow, magenta, cyan, and black--another layer is laminated to the proof and given an all-over exposure to harden it.
This final coating is very expensive as the photosensitive laminate costs about $1.00 per square foot. Also, the hardened polymer does not give longtime protection, because it tends to discolor with age and is easily scratched.
The TRANSFER-KEY system of 3M Company, uses a negative film rather than the positive color film of the DuPont system. In the TRANSFER-KEY system a color emulsion is laminated to proof stock, the negative film is laid over the photosensitive colored emulsion and exposed through ultraviolet light which hardens the photosensitive material in the image parts. After the negative is removed, the unexposed portion of the colored emulsion is washed off the stock leaving the colored image portion firmly adhered to the proof stock. This process is then repeated for each color in the proof. The final image then usually is a four-color image with no protection at all on the color hardened emulsions, which are susceptible to scratching and marring.
It has been proposed to apply a heat sensitive polyethylene or polyester sheet over the proof and heat fuse it to the surface of the proof. Although this is better and more inexpensive than the use of a photosensitive coating, it still is not completely satisfactory.
In the patent art, the patent to Fulwiler U.S. Pat. No. 4,077,830 dated Mar. 7, 1978 uses a pressure sensitive coated plastic sheet to protect movie film and to extend the life of the film. The protective film of Fulwiler has both surfaces finished to less than 10-millionths of an inch RSM on both sides and has a total thickness of less than 0.004 inches.
Another patent of some significance is King U.S. Pat. No. 3,697,277 issued Oct. 10, 1972. The King patent relates to a photographic print with a polyester coating adhered thereto. The polyester has a non-glare or matte surface. The composite wet photographic print-polyester film is heat treated at 190.degree.-260.degree. F. and dried to secure the matte surface to the image layer. Other patents of interest include Stone U.S. Pat. Nos. 3,397,980 and Sharp 3,519,423.
The present invention comprises a pre-press or pre-plate colored proof having a protective coating over the surface thereof supplied by a clear hard plastic sheet having a pressure sensitive backing thereon. The present invention further relates to processes for making said proofs.